Superconducting thin film of compound oxide and process for preparing the same

ABSTRACT

A superconducting thin film of Bi-containing compound oxide deposited on a substrate, a buffer layer made of Bi 2  O 3  being interposed between the superconducting thin film and the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a superconducting thin film of compoundoxide and a process for preparing the same. More particularly, itrelates to an improved superconducting thin film of Bi-containingcompound oxide deposited on a substrate and a process for preparing thesame.

2. Description of the Related Art

When oxide superconductors are utilized in electronics devices or thelike, it is indispensable to prepare their thin films. Thin films of thecompound oxides deposited on single crystal substrates of oxides such asSrTiO₃, MgO or the like exhibits relatively higher superconductingproperty because of the following reasons:

Firstly, bad influence of diffusion or migration of elements of whichthe substrate is made is relatively lower, although such diffusion ormigration of substrate elements is inevitable when the compound oxidesare deposited on substrates.

Secondly, it is rather easy to prepare well-oriented or well-orderedthin films of polycrystals or single crystals of superconductingcompound oxides when these compound oxides are deposited onpredetermined planes of such single crystal substrates of SrTiO₃, MgO orthe like. In fact, so-called epitaxial growth is rather easily realizedwhen the thin films of compound oxides are deposited on such singlecrystal substrates of SrTiO₃, MgO or the like, so that thesuperconducting thin films deposited on these substrates become singlecrystals or polycrystals which have very well ordered crystallinestructure and show improved superconducting property. Still more,anisotropy of the superconducting property which is inherent in thesecompound oxide superconductors can be controlled.

Film formation of these oxide superconductors is effected usually byphysical vapour deposition technique such as sputtering, ion-plating andchemical vapour deposition technique such as MO--CVD.

However, oxide superconductor thin films prepared by known processes aredifficultly applicable to electronics devices because their surfaces arenot smooth. Such surface unevenness of superconductor thin film may becaused by influence of surface roughness of the substrates and bymismatch of lattice constants between substrate crystal andsuperconductor crystal.

Namely, the surface of the single crystal substrate of oxide is notsmooth in the atomic level. In fact, even if the surface of the singlecrystal substrate of oxide is polished completely, its diffractionpattern observed by a reflective high-energy electron diffractionanalyzer (RHEED) is a spotty pattern but is not a streaky pattern whichreflect surface smoothness. The mismatch of lattice constants betweensubstrate crystal and superconductor crystal is another cause of surfaceunevenness because a stress in the thin film which can be absorbed at anearly stage of film formation is released when the thin film becomesthicker.

U.S. Pat. No. 4,837,609 proposes to insert a layer of W, Mo or Tabetween a superconducting compound oxide layer and a silicon singlecrystal substrate.

Japanese patent laid-open No. 63-239,840 proposes to oxidize a coppersubstrate to form a CuO layer thereon and then a superconductingcompound oxide layer is deposited on the CuO layer.

These prior arts, however, neither describes not mentions surfacesmoothness of the superconducting thin films.

Therefore, an object of the present invention is to solve the problemsof known processes and to provide an improved oxide superconductor thinfilm having a smooth surface.

SUMMARY OF THE INVENTION

The present invention provides a superconducting thin film ofBi-containing compound oxide deposited on a substrate, characterized inthat a buffer layer made of Bi₂ O₃ is interposed between thesuperconducting thin film and the substrate.

The Bi-containing compound oxide can be any known compound oxidecontaining bismuth. Followings are examples thereof:

    Bi.sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.x              ( 1)

in which "n" and "x" are numbers each satisfying a range of 1≦n≦6 and6≦x≦16

    Bi.sub.4 (Sr.sub.1-x, Ca.sub.x).sub.m Cu.sub.n O.sub.p+y   ( 2)

in which "x", "m", "n", "p" and "y" are numbers each satisfying a rangeof 6≦m≦10, 4≦n≦8 0<x<1 and -2≦y≦+2, respectively and p=(6+m+n).

In this system, following compositions are preferable:

(i) 7≦m≦9, 5≦n≦7 0.4<x<0.6

(ii) 6≦m≦7, 4≦n≦5 0.2<x<0.4

(iii) 9≦m≦10, 7≦n≦8 0.5<x<0.7

    Bi.sub.2-7 Pb.sub.y Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.x   ( 3)

in which "y", "n" and "x" are numbers each satisfying a range of0.1≦y≦1, 1≦n≦6 and 6≦x≦16 respectively.

These Bi-containing compound oxides are preferably of a single crystal.Thickness of these Bi-containing compound oxides is not limited to aspecial value but is preferably in the order of 100 Å to 1 μm.

The substrate is preferably a single crystal substrate of oxide such asMgO, SrTiO₃ and YSZ in order to facilitate epitaxial growth of thebuffer layer of Bi₂ O₃ and/or of the superconducting thin film. Theother substrates which can be used in the present invention includesingle crystals of LaGaO₃, NdGaO₃ and LaAlO₃.

The film forming plane depends to the substrate used. In the case ofsingle crystal substrates of MgO and SrTiO₃, their {100} plane and {110}plane are preferably used.

An essence of the present invention resides in that a buffer layer madeof Bi₂ O₃ is interposed between the superconducting thin film and thesubstrate.

The buffer layer made of Bi₂ O₃ is preferably of single crystal.Thickness of this buffer layer of Bi₂ O₃ is preferably between 10 Å to1000 Å, more preferably between 10 Å to 100 Å. If the thickness of thisbuffer layer is not thicker than 10 Å, satisfactory advantage of thepresent invention can not be obtained. To the contrary, if the thicknessof this buffer layer exceeds 1,000 Å, crystallinity of Bi₂ O₃ in thethin film become disordered and result in that a bad influence is givento the superconducting thin film. The best crystallinity of Bi₂ O₃buffer layer is realized in a range of thickness between 10 Å to 1,000 Åand a better advantage of the present invention is obtained in thisrange.

The Bi₂ O₃ buffer layer according to the present invention functions toabsorb unevenness of surface roughness of the substrate and to absorbthe difference in lattice constant of crystals between the compoundoxide superconductor and the substrate. And hence, the superconductingthin films of compound oxides according to the present invention havesmooth surfaces which are advantageously applicable to electronicsdevices.

In fact, following points are mentioned as advantages of the presentinventions:

(1) In the superconductors of Bi-containing compound oxidescharacterized by their stratified crystal structures, it is known thatdiffusion or migration of elements between adjacent layers of Ba-O isvery small or reduced. This means that the thin film of Bi₂ O₃ bufferlayer which exists beneath a surface of the Bi-containing compound oxidelayer will not change stoichiometry in the superconducting thin filmbecause of the reduced diffusion or migration.

(2) It is relatively easy to realize epitaxial growth of the Bi₂ O₃ thinfilm on the single crystal substrate. In fact, the surface of the thinfilms of Bi₂ O₃ deposited on the single crystal substrates are so smooththat their diffraction pattern observed by a reflective high-energyelectron diffraction analyzer (RHEED) show streaky patterns.

(3) The lattice constant of Bi₂ O₃ crystal is very similar to that ofBi-containing oxide superconductor such as Bi₂ Sr₂ Ca_(n-1) Cu_(n) O_(x)or the like and hence epitaxial growth of the Bi-containing oxidesuperconductor on the Bi₂ O₃ thin film is facilitated.

Both of the Bi₂ O₃ buffer layer and the superconducting Bi-containingcompound oxide layer can be prepared by any one of known conventionalthin film forming techniques including physical vapour deposition suchas molecular beam epitaxial growth (MBE), sputtering, ion-beamsputtering, and ion-plating and chemical vapour deposition (CVD).

In practice, a thin film of Bi₂ O₃ is firstly deposited on a substratein a vacuum chamber, and then a desired superconducting thin film ofBi-containing compound oxide is deposited on the thin film of Bi₂ O₃ inthe same vacuum chamber.

Since both of the Bi₂ O₃ and the Bi-containing compound oxide areoxides, it is necessary to supply oxygen in addition to metal elementsof which the Bi-containing compound oxide is made during the filmforming stage. The oxygen gas is supplied directly in the neighborhoodof a surface of the substrate while the metal elements are fed in a formof vapors from evaporation source(s). Of course, an oxide or oxides ofconstituent elements of the Bi-containing compound oxide can beevaporated directly from evaporation source(s). Or, any combination ofthese operation modes can be used. The oxygen is preferably activated bymicro-wave radiation before use. Ozone may be used in place of oxygen.It is also effective to produce activated oxygen by creating a plasmadischarge by means of high-frequency in a vacuum chamber.

Film forming conditions of the Bi-containing compound oxides are knownand can be used in the process according to the present invention.Examples of film forming conditions of the Bi₂ O₃ buffer layer are shownbelow but the scope of the present invention should not be limitedthereto.

Examples of Film Forming Conditions of Bi₂ O₃ Buffer Layer

    ______________________________________                                        (1)   In vacuum deposition process,                                                 Pressure in a chamber:                                                                             5 × 10.sup.-4 Torr                                 Substrate temperature:                                                                             550° C.                                           Deposition rate:     1 Å/sec                                              Oxygen supply:       30 ml/min                                                RF powder:           200 W                                              (2)   In sputtering process,                                                        Target:              Bi.sub.2 O.sub.3                                         Substrate temperature:                                                                             600° C.                                           Sputtering gas:      Ar + O.sub.2                                                                  (O.sub.2 is 20 vol %)                                                         5 × 10.sup.-2 Torr                                 Sputtering rate:     0.5 Å/sec                                            Oxygen supply:       30 ml/min                                          ______________________________________                                    

The superconducting thin films according to the present inventionpossess very smooth surfaces and hence improved superconductingproperties, so that they can be utilized advantageously in applicationsof Matisoo switching elements, Annaker memories, Josephson device, avariety of sensors or Superconducting Quantum Interference Device(SQUID) or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an apparatus for depositing thin films of Bi₂ O₃ andof oxide superconductor on a substrate which can be used in the processaccording to the present invention.

FIG. 2 is a RHEED pattern of a thin film of Bi₂ Sr₂ Ca_(n-1) Cu_(n)O_(x) which was grown to a thickness of 500 Å in an example according tothe present invention.

Now, the present invention will be described with reference to Examples,but the scope of the invention should not be limited to the Examples.

EXAMPLE 1

A superconducting thin film of compound oxide according to the presentinvention of Bi₂ Sr₂ Ca₂ Cu₃ O_(x) was deposited on a {100} plane of aMgO single crystal substrate by a molecular beam epitaxy (MBE) unitillustrated in FIG. 1.

The MBE unit shown in FIG. 1 comprises a chamber 1 interior of which isevacuated to a high vacuum, a plurality of Kunudsen Cells (K-cells) 2each of which can control a temperature of a vapour source 10 placedtherein, a plurality of shutters 8 for controlling the amount or timingof each vapour source 10, a substrate holder 34 provided with a heater 4for heating a substrate 5, and an oxygen gas supply pipe 6 through whichoxygen excited by microwave discharge supplied from a microwave source7.

At first, a thin film of Bi₂ O₃ was deposited on the {100} plane of aMgO single crystal substrate under the following conditions:

    ______________________________________                                        Vapour source:         elemental Bi                                           Temperature of the vapour source:                                                                    530° C.                                         Substrate temperature: 600° C.                                         Micro-wave power:      100 W                                                  Deposition rate:       0.5 Å/sec                                          O.sub.2 partial pressure:                                                                            5 × 10.sup.-6 Torr                               Thickness of thin film:                                                                              40 Å                                               ______________________________________                                    

The resulting thin film of Bi₂ O₃ was observed by a RHEED analyzer toshowed a streaky pattern which revealed that the Bi₂ O₃ thin film is anepitaxially grown film of good quality.

Then, a thin film of Bi₂ Sr₂ Ca₂ Cu₃ O_(x) was deposited on the Bi₂ O₃thin film in the same chamber as above under the following conditions:

    ______________________________________                                        Vapour sources and temperatures:                                                                     Bi (530° C.)                                                           Sr (900° C.)                                                           Ca (950° C.)                                                           Cu (1,400° C.)                                  Substrate temperature: 700° C.                                         Deposition rate:       0.5 Å/sec                                          Power of microwave generator:                                                                        100 W                                                  Partial oxygen pressure:                                                                             5 × 10.sup.-6 Torr                               Thickness:             500 Å                                              ______________________________________                                    

FIG. 2 is a diffraction patter of the resulting superconducting thinfilm observed by a RHEED analyzer. The diffraction pattern is so streakythat it reveals such a fact that the superconducting thin film is asingle crystal having a smooth surface.

As a comparison, another thin film of Bi₂ Sr₂ CaCu₃ O_(x) was depositeddirectly on a {100} plane of a MgO substrate under the same condition asabove without the Bi₂ O₃ buffer layer.

The superconducting properties of the resulting superconducting thinfilms according to the present invention and of the comparative examplewere determined. The results are summarized in the following table.

    ______________________________________                                                         Invention                                                                            comparative                                           ______________________________________                                        Critical temperature (K)                                                                         105      93                                                (a temperature from                                                           which resistance could                                                        not be measured)                                                              Critical current density (A/cm.sup.2)                                                            4.2 × 10.sup.6                                                                   1.3 × 10.sup.4                              (at liquid nitrogen                                                           temperature)                                                                  ______________________________________                                    

It was confirmed that the superconducting thin film of Bi-containingcompound oxide shows higher critical temperature and higher criticalcurrent density than those that are obtained by the comparative example.

EXAMPLE 2

Example 1 was repeated by conditions were modified as following:

At first, a thin film of Bi₂ O₃ was deposited on a {100} plane of aSrTiO₃ single crystal substrate under the following conditions:

    ______________________________________                                        Vapour source:         elemental Bi                                           Temperature of the vapour source:                                                                    530° C.                                         Substrate temperature: 600° C.                                         Deposition rate:       0.5 Å/sec                                          Micro-wave power:      100 W                                                  O.sub.2 partial pressure:                                                                            5 × 10.sup.-6 Torr                               Thickness of thin film:                                                                              100 Å                                              ______________________________________                                    

The resulting thin film of Bi₂ O₃ was an epitaxially grown film of thesame high quality as Example 1.

Then, a thin film of Bi₂ Sr₂ Ca₂ Cu₃ O_(x) was deposited on the Bi₂ O₃thin film in the same chamber as above under the following conditions:

    ______________________________________                                        Vapour sources and temperatures:                                                                     Bi (530° C.)                                                           Sr (600° C.)                                                           Ca (650° C.)                                                           Cu (1,180° C.)                                  Substrate temperature: 700° C.                                         Deposition rate:       0.5 Å/sec                                          Power of microwave generator:                                                                        100 W                                                  Partial oxygen pressure:                                                                             8 × 10.sup.-6 Torr                               Thickness:             1,000 Å                                            ______________________________________                                    

The resulting superconducting thin film was a very smooth film whichshows a streaky pattern.

The critical temperature (Tc) and the critical current density (Jc) areas followings:

    ______________________________________                                        Tc:            108 K                                                          Jc:            4.3 × 10.sup.6 A/cm.sup.2 (at 77 K)                      ______________________________________                                    

We claim:
 1. A superconducting thin film of Bi-containing compound oxidedeposited on a substrate, characterized in that a buffer layer made ofBi₂ O₃ wherein said buffer layer has a thickness between 10 Å and 1,000Å is interposed between said superconducting thin film of Bi-containingcompound oxide and said substrate;wherein said superconducting thin filmof Bi-containing compound oxide has a composition represented by theformula:

    Bi.sub.4 (Sr.sub.1-x, Ca.sub.x).sub.m Cu.sub.n O.sub.p+y

in which "x", "m", "n", "p" and "y" are numbers each satisfying a rangeof 6<m≦10, 4≦n≦8, O≦x≦1 and 2 ≦y≦+2 respectively and p=(6+m+n).
 2. Thesuperconducting thin film set forth in claim 1 wherein said buffer layerof Bi₂ O₃ has a thickness between 10 Å and 100Å.
 3. The superconductingthin film set forth in claim 1 wherein said substrate is of a singlecrystal of an oxide.
 4. A superconducting thin film set forth in claim 3wherein said substrate is a single crystal of an oxide selected from thegroup comprising MgO, SrTiO₃ and YSZ.